If cosa = 3 / 5 and a belongs to (3pi / 2,2pi), then cos (a-pi / 3)=

If cosa = 3 / 5 and a belongs to (3pi / 2,2pi), then cos (a-pi / 3)=

It is known that cosa = 3 / 5, and a belongs to (3 π / 2,2 π) in the fourth quadrant,
There are: Sina = - √ (1-cos & # 178; a) = - 4 / 5;
The results show that cos (a - π / 3) = cosa * COA (π / 3) + Sina * sin (π / 3) = (3 / 5) * (1 / 2) + (- 4 / 5) * (√ 3 / 2) = (3-4 √ 3) / 10

It is proved that Tan α / 2 = ± √ 1-cos α / 1 + cos α,

√(1-COSα)/(1+COSα)=√[1-(1-2*(sina/2)^2]/[1+2*(COSα/2)^2-1]
=√2*(sinα/2)^2/2*(COSα/2)^2=√tanα^2=±tanα/2

If cos (a + β) = 5 / 1, cos (a - β) = 5 / 3, and 0 < a < β < 2 / π, the value of cos2a is obtained

∵0＜a＜β＜π/2
∴0